Thousands of older adults are dying prematurely every year from air pollution – even though the air they’re breathing is legally clean and safe.

A new study by researchers at Harvard University of all 48 million Americans aged 65 and older on Medicare found people were dying after just a single day of breathing air that met federal standards, but was somewhat dirty. The study was published Tuesday in the Journal of the American Medical Association (JAMA).

“The current understanding is, the Environmental Protection Agency is setting safety standards and if pollution is below that standard, everyone is safe,” said data scientist and co-author Francesca Dominici. “That’s basically not what the scientific evidence is saying.”

EPA regulates six airborne pollutants, including fine particles and ozone (or smog). Its air standards are supposed to “protect the health of 'sensitive' populations such as asthmatics, children, and the elderly," according to the agency's website.

For particulate matter, EPA set the safe level of short-term exposure to fine particles at 35 micrograms per cubic meter of air and 70 parts per billion for ozone. But Dominici and her colleagues found that people were dying after breathing air that was much cleaner than that. For every additional unit of ozone in the air, she found an additional 250 seniors died prematurely over the course of that year. It was even worse for particulate matter: 550 additional seniors died annually for each additional unit of particulate matter they were exposed to.

The researchers concluded there is no safe level of exposure to either pollutant.

"It's that simple," Dominici said, "the lower you go, the more lives you save."

Jennifer Ailshire, a professor of gerontology at USC, said the study raised questions about whether current EPA standards actually protect "sensitive" groups.

“I always wonder if the standard that’s set for the entire population is perhaps a little too high for what we would want for older populations,” she said.

Elderly adults are more vulnerable to air pollution because their age gives them a higher chance of having other chronic diseases that are exacerbated by breathing dirty air. Tiny particles of soot, smoke or vehicle exhaust just a fraction the width of a grain of sand can lodge deep inside the lungs or pass into the bloodstream, causing inflammation.

Younger adults or seniors without preexisting conditions like asthma, heart disease or other cardiovascular illnesses can still be affected by air pollution, but “it’s unlikely to result in a hospitalization or death,” said Ailshire.

In greater Los Angeles, air quality routinely exceeds federal standards for ozone and particulate matter. This year has been particularly bad: between June 1 and August 30, 2017, there were just two weeks where ozone levels were considered safe. And December 29 is the fifth day in a row where particulate levels exceeded EPA standards.

Ailshire recommends older adults treat air quality like weather and check it routinely before exercising outdoors.

But Dominici said her study calls for more than just increased personal responsibility, because it is nearly impossible for people to avoid contaminated air. “Everyone is affected by exposure to pollution," she said.